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Date: Wed, 25 Mar 2015 20:06:58 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20150325120658.GA20445@fam-t430.nay.redhat.com>
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Subject: Re: [Qemu-devel] AioContext of block jobs
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

On Wed, 03/25 12:25, Paolo Bonzini wrote:
> 
> 
> On 25/03/2015 09:31, Fam Zheng wrote:
> > I was looking at block jobs' AioContext and realized that the block job
> > coroutines are actually started in main loop.
> > 
> > I'm confused because 5a7e7a0bad17c96e03f55ed7019e2d7545e21a96 and friends in
> > the series [1] seem to move the coroutines to the BDS's iothreads, but it
> > didn't do that.
> > 
> > (Although after the first block_job_yield or sleep, the coroutines ARE resumed
> > in the right AioContext.)
> > 
> > Why is it safe to start the jobs from the main thread where QMP command is
> > handled? I see no guarantee that the jobs won't access BDS before first yield
> > but after releasing the AioContext.
> > 
> > Is this a bug?
> 
> It's okay because the coroutine is started while the main thread is
> holding the AioContext.  So the first "stint" of the coroutine, until
> the first yield, is done in the main thread but still with the
> AioContext held.

I see! That's what I missed. Thanks!

Fam

> 
> After the first yield, the main thread releases the AioContext, the
> dataplane thread gets it back and the coroutine is moved to the
> dataplane thread.
>